sysupgrade on pcEngines apu2 boards hangs

I have been running OpenBSD on three apu2 boards (as firewalls) for several years and doing remote (ssh to boards) with no problems.

Doing the 7.4 to 7.5 upgrades I had an initial success then two failures. Not confidence building since these firewalls are at remote sites.

Not sure where the problem lies, here is what I've experienced:

pcEngines apu2 boards, running sysupgrade - have been running OpenBSD
since about 6.2 and doing remote (ssh to board) for several releases
w/o any problems

Did successful sysupgrade on one board the evening before 7.5 release
notification using: url https://cdn.openbsd.org/pub/OpenBSD/ (other
mirrors not yet showing items in 7.5 directory)

Went fine, as per norm.

Next day did on a second board using my default url (as per all previous
remote upgrades) https://mirrors.sonic.net/pub/OpenBSD/

after syspugrade pings ok but cannot ssh:
ssh: connect to host 10.42.xx.xx port 22: Connection refused

used minicom to hook up to serial port and was able to see (didn't
require login??):

# ls -al
total 166
drwxr-xr-x 11 root wheel 512 Apr 7 18:34 .
drwxr-xr-x 11 root wheel 512 Apr 7 18:34 ..
-rw-r--r-- 1 root wheel 1770 Mar 20 21:53 .profile
-rw-r--r-- 1 root wheel 194 Apr 7 18:34 auto_upgrade.conf
lrwxr-xr-x 1 root wheel 11 Mar 20 21:53 autoinstall -> install.sub
drwxr-xr-x 2 root wheel 512 Mar 20 21:53 bin
drwxr-xr-x 2 root wheel 2560 Apr 7 18:34 dev
drwxr-xr-x 5 root wheel 512 Apr 7 18:34 etc
lrwxr-xr-x 1 root wheel 11 Mar 20 21:53 install -> install.sub
-rw-r--r-- 1 root wheel 2903 Mar 20 21:53 install.md
-rwxr-xr-x 1 root wheel 64483 Mar 20 21:53 install.sub
drwxr-xr-x 15 root wheel 512 Apr 7 18:36 mnt
drwxr-xr-x 2 root wheel 512 Mar 20 21:53 mnt2
drwxr-xr-x 2 root wheel 1024 Mar 20 21:53 sbin
drwxrwxrwt 4 root wheel 512 Apr 7 18:36 tmp
lrwxr-xr-x 1 root wheel 11 Mar 20 21:53 upgrade -> install.sub
drwxr-xr-x 6 root wheel 512 Mar 20 21:53 usr
drwxr-xr-x 6 root wheel 512 Mar 20 21:53 var

reboot from serial console with minicom

Comes back up (can ssh now) but with only one cpu (normally running mp - cpu[0-3] )

reboot

Watching on serial port

reorder_kernel: failed -- see /usr/share/relink/kernel/GENERIC/relink.log

as per log: run 'sha256 -h /var/db/kernel.SHA256 /bsd'

reboot, no error, but still in single cpu mode.

reboot bsd.mp

hang, ssh connection refused

Power cycle to get fresh reboot but still hangs

still on serial port: boot usb with miniroot75.img

reboot - needed to reset - 'stty com0 115200; set tty com0', otherwise hangs

Choose upgrade

Success!

This happened on a third apu2 board; thus, two unsucessful and one successful sysupgrades on apu2 boards

Any ideas/pointers? I'd like this not to reoccur.